似乎越抽象之用語越容易被廣泛使用，即使對其內涵不甚瞭解。民法第一條之「法理」便是一個學說眾說紛紜，不易掌握，卻又被最高法院浮濫使用之民法概念。本文試著先從法理學(色括法哲學及法學方法論)之角度出發，從歷史的發展探究出發，探討民法第一條之「法理」的三個內涵：自然法、事物本質及法律原則，並由此推衍出民法第一條之「法理」所具備之兩個功能：審查或控制法律之功能，以及補充法律之功能。按著從民法釋義學與法理學之間學科關連的角度出發，指出民法第一條之「法理」即為法理學研究之客體，因而賦予法理另一個功能：學科接駁之功能，希望藉此提醒同時民法釋義學及法理學之研究者，應並重兩門學科之研究。 接著從「法理」具體之可能內容分析整理最高法院民事判決判例中所使用之「法理」究竟所指為何。嘗試將這些判決分成三類：第一類是與類推適用無關者，其所指之法理包括法律本身、學說、判例及外國立法例；第二類是類推適用本身，是其所指之法理可能涉及立法理由，主要則指平等原則。本文試著分析類推適用之基本結構，步驟及要素，並藉以評論最高法院類推適用步驟之混亂，為此並試著提出「類推適用標準論述語句」供最高法院實際運用之參考；第三類則是向來學說不重視，但實務常用之輕重相舉之推論，本文歸納整理最高法院共十七個判決的不同論述語句，並試著證立最佳論述語句，同時澄清通說及實務將輕重相舉之推論誤解為當然「解釋」之不當。 最後談一個法理與法律溯及既往之問題，澄清最高法院之判決是透過法理在進行制定法外之法律續造，並非不當地將法律溯及既往，並藉此印證最高法院對民法第一條之一知半解。最終展望本文對民事實務提供方法論之微薄貢獻。

It seems that the more abstract a concept is, the more frequently it is used, although the user doesn't understand it.‘The nature of things’ in §1 civil code is a best example of which was made comprehensive from our supreme court. In this essay I try to observe the history of legal theory (it includes legal philosophy and legal method) in order to induce three meanings of ‘the the nature of things’ in §1 civi1 code, namely the natural law, the nature of things and principles of law. Furthermore I would like to deduce two functions of 'the nature of things' in §1 civil code from the abovementioned three contents, namely the function of control and the function of complement. Besides, I would like to induce the third function of ‘the nature of things' in §1 civil code from the point of view related to civil dogmatic theory as well as jurisprudence, that is function of connection. I hope therefore to emphasize that the research of civil dogmatic theory and jurisprudence in Taiwan is equally important. After the discussion of the abstract contents of ‘the nature of things' in §1 civil code, I try to sort precedents of our Supreme Courts in which the word ‘the nature of things’ has been used. Altogether there are three kinds. The first one is not dealing with analog application, in which ‘the nature of things’ means either law itself or theory or precedents or foreign lawful rule. The other kind points analog application to itself, in which ‘the nature of things’ means the reason of legislation in law or principle of equality. I have analyzed the basic process of analog argument and commented on the confused analog application in our precedents and then I also tried to create a ‘standard arguments sentence’ for our Supreme Court, in order that it can be correctly applied in the future. The third is so called ‘argumentuma minori ad maius’, (argument from small to large) und ‘argumentuma maiori ad minus,’ (argument from large to small). In conclusion, I have deduced 6 different formulations from 17 precedents and justified the best ‘arguments sentence’ of 6 formulations. In the end I have discussed a practical question which dealing with ‘the nature of things’ and retroactive effect of law. I try to make the judgment of the Supreme Court more clearly which is in fact not a matter of lawful retroaction. It concerns a legal further creation through ‘the nature of things’ indeed. The Supreme Court has misunderstood the problem. I hope this essay can submit some contributions to legal methodology in practice.